Scattered and reflected incident radiation can expose a radiation sensitive layer in regions in which exposure is not desired. The use of antihalation layers to prevent this unwanted exposure is well known. Typically these auxiliary layers contain a dye or a pigment which absorbs the incident radiation. An antihalation layer may be either a backing layer, positioned on the side of the support opposite that bearing the radiation sensitive layer or layers, or an undercoat layer, located between the support and the radiation sensitive layer or layers.
Although dyes are used in antihalation layers, they frequently cause undesirable residual stain on the processed film. The use of manganese dioxide in antihalation layers is well known. The material possesses desirable absorption characteristics for use in antihalation layers and is cleared in the developing process.
Manganese dioxide containing antihalation layers have been prepared by a number of different processes. Gelatin, alcohols, and polyvinyl alcohol have each been used to reduce potassium permanganate to manganese dioxide. Mackey, U.S. Pat. No. 3,869,401, discloses the preparation of manganese dioxide by reduction of potassium permanganate with polyols. Sato, U.S. Pat. No. 3,773,539, discloses the reduction of potassium permanganate with a chemically modified gelatin obtained by reacting the gelatin with a compound capable of reacting with the amino group of the gelatin, such as sulfonyl chlorides, carboxylic acid chlorides, acid anhydrides, etc. Hine, U.S. Pat. No. 3,630,739, discloses preparation of manganese dioxide by addition of potassium permanganate to an aqueous solution of a hydrolyzed maleic anhydride copolymer, such as a methyl vinyl ether/maleic anhydride copolymer, an ethylene/maleic anhydride copolymer, etc. In this process the polymer hydrolyzate serves as the reducing agent or an auxiliary reducing agent may be added to assist in the reduction of the permanganate salt.
Each of these processes has certain disadvantages. Some are extremely difficult to run on a commercial scale and/or can produce unstable colloids which aggregate producing materials which are unsuitable for use in antihalation layer. Use of gelatin as the dispersing agent causes degradation of the gelatin due to attack of the permanganate on the gelatin. Aldehydes can not be used as reducing agents with gelatin since they would crosslink the gelatin. Other dispersing agents can produce unstable colloids which are not suitable for use in antihalation layers.
To be useful in an antihalation layer, the pigment must be colloidal in size for optical efficiency and must not form aggregates during the preparation process. In addition, the process must not cause degradation of the properties of the layer, especially if the layer is a backing layer containing matte particles so that it can be used for drafting and other applications in which it is desired to write on the backing layer. Current manganese dioxide containing backing layer suffer from this disadvantage, causing pen clogging, pencil gouging, etc.